Question

Pollen grains are formed in
(a) Pollen tube
(b) Stigma
(c) Filament
(d) Pollen sac

Answer 1

Hint: This structure is present in the plant that holds the pollen grains which is commonly known as microsporangia. It is the structure in seed plants in which pollen is produced.

Complete answer:
Pollen grains represent the male gametophyte. They are formed in pouch-like structures present inside the anther lobes known as pollen sac. Pollen sacs are filled with pollen grains. The pollen tube carries the contents of the pollen grain (i.e. male gametes).
The cell wall of the pollen grain is called sporoderm and is made up of two layers, exine, and intine.
The outer covering of the pollen grain is made up of sporopollenin (one of the most resistant organic materials known) . It can withstand high temperatures, strong acids, and alkali. No enzyme that degrades sporopollenin is known yet.
Pollen grain divides mitotically into two cells, a vegetative cell and a generative cell.
Generative cell divides to form two male gametes and pollen is shed at the three-celled stage.
The pollen grain germinates on the stigma to produce a pollen tube through the germ pore. The contents of the pollen grains move through the pollen tube towards the ovary.
The pollen tube grows through the tissues of the stigma and style and reaches the ovary. The growth of the pollen tube is chemotropic.
After entering the synergids, the pollen tube releases two male gametes into the cytoplasm of the synergids.
The two types of fusion i.e. syngamy and triple fusion. It is termed as double fertilization.
In syngamy, the male gamete (n) moves towards the egg cell (n) and fuses with its nucleus. Thus, resulting in the formation of a diploid cell, the zygote.
In triple fusion, the male gamete (n) moves towards the two polar nuclei (2n) to form a triploid primary endosperm nucleus.
So, the answer is ‘Pollen sac’.

Note: There are mainly four pollen sacs in each anther that contain the microspore mother cells in angiosperms, however, In gymnosperms different numbers of pollen sacs are borne on the microsporophylls that make up the male cone.